The present invention relates to an apparatus for optically inspecting an object, comprising an object carrier for carrying the object, a pattern generating unit for illuminating the object with a measurement pattern, an image capture unit for capturing a number of images of the object, an imaging optics for influencing a light beam path between the object and the image capture unit, and a data processing unit, which is designed to determine at least one property of the object on the basis of the number of images.
In accordance with a further aspect, the present invention relates to a method for changing an operating mode of an apparatus for optically inspecting an object.
In the industrial manufacture of products, the product quality has increased in importance for many years. High product quality can be achieved, firstly, by means of appropriately designed and stable manufacturing processes. Secondly, the quality parameters of a product have to be monitored as reliably and fully as possible in order to identify quality deficiencies at an early stage. In many cases, the quality of a product surface is important. This can involve decorative surfaces, such as, for example, paint surfaces in the case of motor vehicles or domestic particles, or technical surfaces, such as, for instance, the surfaces of precision-machined metallic pistons or bearing surfaces.
There are already a large number of proposals and designs for inspecting surfaces.
The document DE 10 2009 021 733 A1 describes for example a deflectormetric method and a corresponding apparatus. This method involves projecting a stripe pattern having a sinusoidal brightness profile onto a screen arranged obliquely above a surface to be inspected. The projected pattern is varied or moved, such that correspondingly varied stripe patterns are incident on the surface. During or after the variation/movement of the pattern, in each case an image of the surface is captured with the reflected pattern. By means of a mathematical combination of the images captured at different points in time, the intention is to generate a result image on the basis of which defective regions and defect-free regions of the surface can be distinguished computationally and/or visually.
Further deflectometry methods are disclosed for example in the documents DE 10 2008 038 256 A1 and DE 10 2008 064 562 A1.
Furthermore, so-called fringe pattern methods or stripe projection methods are known in the prior art. By way of example, the document DE 10 2010 007 922 A1 discloses the use of such a fringe pattern method. In this case, a stripe pattern is projected onto the object to be inspected, for example by an illumination device being directed at the object through a multi-stripe grating. Bright and dark stripes alternate in the multi-stripe grating. The width of the stripes of the multi-stripe grating determines—together with an angle between illumination direction and observation direction—the accuracy or the resolution of the three-dimensional detection of the object. From the position of an illumination device of the multi-stripe grating and the position of the multi-stripe grating, it is possible to calculate the position of a light plane running from the illumination device through the stripes of the multi-stripe grating. On the basis of the position of a pixel in the image of the first image capture device, it is in turn possible to calculate the vector of a light beam that generated said pixel. This in turn makes it possible to determine the point of intersection of said light beam with the calculated plane of the multi-stripe grating. The spatial coordinates of a specific pixel on the image of the image capture unit are thus obtained.
Provision can be made for the stripe projection method to be a Graycode stripe projection method. In principle, bright and dark stripes merely alternate in a multi-stripe pattern. This means, however, that a for example bright pixel in an image of the first image capture device cannot be given an absolute assignment to one of said stripes. Therefore, a so-called Graycode method is used to enable a unique assignment. In this case, by way of example, firstly only one bright and one dark stripe are projected onto the face, in a second step each of said stripes is in turn subdivided into one bright and one dark stripe, such that a total of four stripes are present, in a next step a subdivision is in turn effected, such that eight stripes are present, etc. The subdivision is effected until finally the desired stripe width or resolution is present. If an image of the object is then captured for each stripe resolution, it is possible, on the basis of the bright/dark change, for each pixel to be uniquely assigned a stripe of the stripe grating represented last with the desired resolution.
A further refinement of a fringe pattern method can be a phase shift method, which enables a resolution in a subpixel range. In this case, a sinusoidal brightness profile is modulated on the stripe pattern, which is rectangular per se with regard to its brightness values. A first image capture is then effected, and the phase of the modulated wave is subsequently shifted by π/2 transversely with respect to the beam direction. This is followed by renewed capture and renewed shifting until a total of at least four images have been captured. From the four brightness values of the pixel in the four captured recordings it is possible to deduce its phase angle within the modulated signal. The exact position of the pixel within a stripe thus becomes determinable.
Further fringe pattern methods are disclosed for instance in the documents DE 10 2008 041 343 A1 and EP 2 327 956 A1.
It is often the case, however, that the known methods and apparatuses can be used only for a specific application, since they presuppose a high level of prior knowledge about the surface to be inspected. Furthermore, alongside a reliable inspection of surfaces, it is also necessary to comply with industrial conditions such as complying with cycle times relevant to incorporation into industrial manufacturing, the capability of carrying out the surface inspection in a factory, and/or the possibility of adapting the surface inspection to changing products simply and rapidly.
What the fringe pattern method and the deflectometry method have in common is that they use stripe patterns having different stripe geometries with regard to stripe width, gap width, duty ratio of stripe and gap, direction or orientation and phase angle.
During use, a fringe pattern method is particularly sensitive to inclinations in a surface of the object and is therefore particularly well suited to identifying topographies. By contrast, a deflectometry method is particularly suitable for identifying depressions and defects in a surface. Deflectometry systems have advantages in use in the case of very smooth, highly reflective to mirroring surfaces. In the case of a surface that is rather rough and reflects incident light less well, that is to say has rather a scattering or absorbing effect, the use of a stripe projection method is generally advantageous.
Against this technical background it is therefore an object of the present invention to specify an apparatus for inspecting an object and a method which eliminate the disadvantages outlined and to enable more variable inspection, in particular of changing objects.